1. Field the Invention
The present invention generally relates to an image forming device. More specifically, the present invention relates to an image forming device having a data storage unit that stores image data.
2. Background Information
In image forming devices, resolution enhancement technology is used in which a dot to be printed is resolved to ¼ or ⅛, and jaggedness that occurs in sloping lines or the like is corrected so that the jaggedness does not stand out. This resolution enhancement is disclosed in Japanese Patent No. 3275050, which was registered on 8th Feb. 2002 (see especially column 26 line 49 to column 27 line 16, FIG. 17).
In the enhanced resolution method, an artificial latent image shown by the broken line in FIG. 4 is interleaved between two actual latent images shown by the solid lines. Therefore, as shown in FIG. 5, artificial latent image lines 2 known as interleave scan lines are disposed in a position between actual latent image lines 1 known as physical scan lines. By interleaving the artificial latent image between actual latent image and actual latent image the resolution is increased. Note that, in FIG. 4, the horizontal axis shows scanning position and the vertical axis shows exposure intensity.
As shown in FIG. 4, the artificial latent image line is a pseudo line which is formed by overlapping exposure of two actual latent image lines in the slow scan direction. The exposure intensity of the aforementioned two actual latent image lines is adjusted properly in order to form the artificial latent image line.
However, in image forming devices it is possible to select a mode in which characters and the like are printed on both sides of a sheet. For example, as shown in FIG. 6A, on the first sheet of a rectangular document 4 the characters “ABCD” are written parallel to the short edge which is the first scan direction. Then, as shown in FIG. 6B, on the second sheet of the document 5 the characters “EFGH” are written parallel to the same short edge. An image forming device reads the document 4 and the document 5 in the slow scan direction, indicated by the arrows, that is perpendicular to the short edge. Then, as seen in FIG. 6C, the image forming device prints the characters “ABCD” of the document 4 on a printing sheet 6. Also, the image forming device carries out double sided printing by for example reversing the sheet 6 in the same slow scan direction, and printing the characters “EFGH” of the document 5.
By carrying out double-sided printing in this way, the characters printed on a first side and the second side of the sheet 6 are reversed in the vertical direction, as shown in FIG. 6C. If the sheet 6 that has been printed on both sides in this way is bound on one short edge (hereafter referred to as short edge binding), the sheet 6 is turned about the one short edge as center. Therefore when the sheet 6 is turned, the characters on the second side will appear normally in the vertical direction.
When bound along a long edge, the sheet 6 is turned about the long edge as center. In addition, when the sheet 6 is turned, the characters on the second side are reversed in the vertical direction with respect to the characters on the first side. To prevent this reversal, it is necessary to reverse the characters on the second side in the vertical direction. In other words, it is necessary to reverse the sweep direction of the characters on the second side.
In an image forming device such as a digital multi-function printer or the like, the image data read from documents such as those shown in FIG. 6A and FIG. 6B are temporarily stored in memory or the like. Assuming it is not possible to change the direction of transport of the sheets in the printer, when printing in the long edge binding mode using image data as described above, it is necessary to read the image data on the first side from the top in the slow scan direction, and read the image data on the second side from the bottom.
The following is an explanation of printing an A4 sheet with long edge binding using 1200 dpi resolution enhancement, using this type of image data as an example. An A4 sheet has a rectangular shape whose size in the respective horizontal and vertical directions is 210 mm by 297 mm. If a margin of 5 mm is allowed at the top and bottom of an A4 sheet, the scan area from one end to the other end in the vertical direction is 287 mm in the slow scan direction. One inch is 25.4 mm, so the number of scan lines is 1200×287/25.4=13,359.055, so 13,360 scan lines are required.
FIGS. 7A and 7B diagram the 13,360 lines of this image data. For convenience of explanation, the image data of FIG. 7B is the same as the image data of FIG. 7A but reversed in the vertical direction. Assume that FIGS. 7A and 7B show the dots for printing a slanted line. As shown in FIG. 7A, the image data include scan lines 1 through 13,360. When the total number of scan lines is an even number, the actual latent image lines and the artificial latent image lines become interchanged when reversed in the vertical direction, as shown in FIG. 7B.
In other words, as shown in FIG. 7A, when printing image data in the forward direction (here, one direction is referred to as the forward direction, and the opposite direction is referred to as the reverse direction), odd numbered lines are actual latent image lines, and even numbered lines are artificial latent image lines. Here, for example, focus on the last line, number 13,360, of FIG. 7A. This line is an even numbered line so it is an artificial latent image line. In contrast to this, when reversed in the vertical direction and printed, as shown in FIG. 7B, the line corresponding to line 13,360 in FIG. 7A is line 1 in FIG. 7B. In other words, the line has become an odd numbered line so that it is an actual latent image line. In this way, as shown in FIG. 7A, odd-numbered lines of the image data that are read in the forward direction and drawn as actual latent image lines become even-numbered lines in FIG. 7B and are drawn as artificial latent image lines. Even-numbered lines that are drawn as artificial latent image lines in FIG. 7A become odd-numbered lines and are drawn as actual latent image lines in FIG. 7B. However, there is a problem that the density and tone of an image as a whole changes depending on whether a part of the same image is drawn as an actual latent image or as an artificial latent image.
The actual latent image lines are in positions corresponding to physical scan lines based on the actual data. In contrast, the artificial latent image lines are located at interleaved scan lines between physical scan lines. If the same dot is printed, the density and tone of the actual latent image and the artificial latent image will be subtly different.
In view of the above, it will be apparent to those skilled in the art from this disclosure that there exists a need for an improved image forming device. This invention addresses this need in the art as well as other needs, which will become apparent to those skilled in the art from this disclosure.